Generation of cardiomyocytes from human pluripotent stem cells

ABSTRACT

Methods for generating high-yield, high-purity cardiomyocyte progenitors or cardiomyocytes from pluripotent cells are described. Wnt/β-catenin signaling is first activated in pluripotent cells, e.g., by inhibition of Gsk-3 to obtain a first population of cells. Wnt/β-catenin signaling is then inhibited in the first cell population to induce cardiogenesis under fully defined, growth factor free culture conditions.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims priority to U.S. provisional patentapplication No. 61/546,686, filed on Oct. 13, 2011, which isincorporated by reference in its entirety as if set forth herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under EB007534 awardedby the National Institutes of Health and EFRI-0735903 awarded by theNational Science Foundation. The government has certain rights in theinvention.

BACKGROUND

Generating cardiovascular cells from pluripotent stem cells holds greatpromise for cardiovascular research and therapy. However, cardiogenesisis regulated by numerous developmental pathways. Moreover,differentiation of pluripotent stem cell into cardiac cells isinefficient and results in heterogeneous cultures, limiting theusefulness of this approach. Pluripotent stem cells, such as humanembryonic stem (hES) cells and induced pluripotent stem (iPS) cells,collectively human pluripotent (hPS) cells can perpetually proliferateand differentiate into derivatives of all three embryonic germ layers(Thomson et al., Science 282:1145 (1998); Odorico et al., Stem Cells19:193 (2001); Yu et al., Science 318(5858):1917 (2007)). Pluripotentstem cell cultures can differentiate spontaneously, yielding a seeminglyrandom variety of cells (Watt and Hogan, Science 287:1427 (2000)). Theearliest pluripotent stem cell differentiation methods allowed stem cellaggregates to spontaneously differentiate and form embryoid bodies (EBs)which contain precursors of the three primary germ layers, including, insome cases, cardiomyocytes. Such methods are inefficient, however, asonly few percent of the developing cells become cardiomyocytes.

More recent methods direct differentiation of ES and iPS cells(pluripotent cells generated by reprogramming somatic cells ordifferentiated progenitor cells to pluripotency) into cardiomyocyteswithout EB formation by sequentially applying various combinations ofsoluble, exogenous growth factors and small molecules to mimic cardiacdevelopment. Soluble factors important for embryonic cardiac developmentinclude Activin A, BMP4, nodal, Wnt agonists and antagonists, bFGF andother molecules (Conlon et al., Development 120(7):1919 (1994); Lough etal., Dev. Biol. 178(1):198 (1996); Mima et al., PNAS 92(2):467 (1995);Zaffran and Frasch, Circ. Res. 91 (6), 457 (2002)). The addition ofFGF2, Activin A, BMP4, DKK1 and VEGF, can enhance cardiomyocytedifferentiation in embryoid bodies (EBs) (Yang et al., Nature453:524-528 (2008)). However, this protocol is labor-intensive and notapplicable to all pluripotent cell lines since it requires monitoring ofKDR/c-kit (Yang et al., Nature 453:524-528 (2008)) or Flk1/PDGFRα(Kattman et al., Cell Stem Cell 8:228-240 (2011)) expression andoptimization of growth factor concentrations for efficient cardiacdevelopment in various hPSC lines. Protocols for cardiomyocyteprogenitor and cardiomyocyte differentiation that do not require cellline-specific optimization are desirable. Identification of definedfactors that promote cardiomyocyte progenitor and cardiomyocytedifferentiation has enabled development of monolayer-based directeddifferentiation protocols, such as, sequential treatment of Activin Aand BMP4, which has been reported to generate greater than 30%cardiomyocytes in the H7 hESC line (Laflamme et al., Nat. Biotechnol.25:1015-1024 (2007)). However, the efficiency of the Activin A and BMP4directed differentiation protocol can be highly variable between celllines and experimental repeats (Paige et al., PLoS One 5: e11134(2010)).

Apart from their somatic cell origin, iPS cells share manycharacteristics of embryonic stem cells, such as the ability to growperpetually and to differentiate into cells of all three germ layers.Like ES cells, iPS cells express pluripotency markers, such as OCT-4,SSEA-3, SSEA-4, TRA-1-60, TRA-1-81, and Nanog. iPS cells have beengenerated using retroviral vectors that randomly insert exogenous DNAinto the target cell genome. Vector- and transgene-free iPS cells havebeen generated by using non-integrating vectors. Using non-integratingvectors avoids the risk of aberrant cellular gene expression andneoplastic growth (Okita et al. Nature 448:313 (2007)). Loss of thereprogramming vector also avoids perpetual expression of transgenes thatcan induce programmed cell death (apoptosis) (Askew et al., Oncogene6:1915 (1991), Evan et al., Cell 69:119 (1992)) and interfere withsubsequent differentiation of iPS cells.

More recently, methods were devised for reprogramming somatic cellsusing oriP/Epstein-Barr nuclear antigen-1 (EBNA1)-based episomal vectorsthat do not integrate into the genome and are lost from the cells afterreprogramming (Yu et al., Science 324(5928):797 (2009)). iPS cellsgenerated by this method are vector- and transgene-free and, as such,are well suited for clinical application. However, vector-free iPS cellshave not yet been demonstrated to differentiate intoclinically-applicable cardiomyocyte progenitors and cardiomyocytes withhigh efficiency (e.g., >90%) under fully defined conditions (both mediumand substrate are defined).

Wnt proteins control morphogenesis, and are involved in development,stem cell differentiation control, and cell malignant transformation.The Wnt signaling pathway is a key regulator of cardiogenesis in vivoand in vitro. In chick and frog embryos, canonical Wnt signalingrepresses early cardiac specification (Marvin et al., Genes Dev. 15,316-327 (2001); Schneider and Mercola, Genes Dev. 15, 304-315 (2001);Tzahor and Lassar, Genes Dev 15, 255-260 (2001)) and has a biphasiceffect in zebrafish, mouse embryos, and mouse embryonic stem cells(Naito et al., Proc. Natl. Acad. Sci. U.S.A. 103, 19812-19817 (2006);Ueno et al., Proc. Natl. Acad. Sci. U.S.A. 104, 9685-9690 (2007), withearly Wnt signaling enhancing cardiogenesis and later signalingrepressing heart development. Endogenous Wnt signaling is also requiredafter treatment to differentiate hES cells to cardiomyocytes withActivin A and BMP4 (Paige et al., PLoS One 5: e11134 (2010)). However,it remains unknown whether differentiation of human pluripotent stemcells to cardiomyoctes conserves such stage-specific Wnt signalingroles.

The canonical Wnt pathway describes a series of events that occur whenWnt ligands bind to cell-surface receptors of the Frizzled family,causing the receptors to activate Dishevelled family proteins, resultingin a change in the amount of β-catenin that reaches the nucleus.Modulation of Gsk3 and Wnt pathway signaling triggers expression of avariety of developmental cues (e.g. Nodal (Kattman et al., Cell StemCell, 8:228-240 (2011)), BMP2/4 (Kattman et al., 2011; Laflamme et al.,Nat. Biotech 25:1015-1024 (2007)), Noggin (Ma et al., Cell Res.21:579-587 (2011)), WNT3a (Tran et al., Stem Cells, 27:1869-1878 (2009))and WNT8a (Paige et al., PLoS One 5: e11134 (2010)) and transcriptionfactors involved in cardiomyocyte differentiation (e.g. T (Asashima etal., Faseb J. 23:114-122 (2009)) and MIXLJ (Davis et al., Blood111:1876-1884 (2008)), ISL1 (Bu et al., Nature 460:113-117 (2009) andNKX2-5 (Lints et al., Development 119:969 (1993)), TBX5 (Bruneau et al.,Dev Biol 211:100-108 (1999)), GATA4 (Kuo et al., Gene Dev 11:1048-1060(1997a); Kuo et al., Circulation 96:1686-1686 (1997), and MEF2C(Edmondson et al., Development 120:1251-1263 (1994)).

There is in the art a need for a cardiac differentiation protocol thatuses completely defined, growth factor-free culture conditions toproduce cardiomyocyte progenitors and cardiomyocytes from hPS cells.

BRIEF SUMMARY

The invention relates generally to methods for cardiac induction in hPScells and, more particularly, to methods for generating, from hPS cells,populations of cardiomyocyte progenitors, which go on to becomefunctional cardiomyocytes under chemically-defined, growth factor-freeconditions by sequential activation and inhibition of Wnt/β-cateninsignaling.

Accordingly, in one aspect provided herein is a method for generating apopulation of cardiomyocyte progenitors from pluripotent stem cells,comprising: (i) activating Wnt/β-catenin signaling in culturedpluripotent stem cells (e.g., primate pluripotent stem cells, humanpluripotent stem cells, or non-human primate pluripotent stem cells) toobtain a first cell population; (ii) culturing the first cell populationfor a period following the end of the activating step; and (iii)inhibiting Wnt/β-catenin signaling in the cultured first cell populationafter the culturing period in step (ii) to obtain a second cellpopulation comprising cardiomyocyte progenitors.

In some embodiments of the just-mentioned method, activating theWnt/β-catenin signaling comprises inhibiting Gsk3 in the pluripotentstem cells. In some embodiments, Gsk3 in the pluripotent stem cells isinhibited by contacting the pluripotent stem cells with a small moleculethat inhibits Gsk3. In some embodiments, the small molecule inhibitorthat inhibits Gsk3 is CHIR 99021, CHIR 98014, BIO-acetoxime, BIO, LiCl,SB 216763, SB 415286, AR A014418, 1-Azakenpaullone, andBis-7-indolylmaleimide, or a combination thereof. In another embodiment,the small molecule Gsk3 inhibitor to be used is CHIR 99021, CHIR 98014,and BIO-acetoxime. In one embodiment, the small molecule Gsk3 inhibitorto be used is CHIR 99021.

In some embodiments, inhibiting Gsk3 in the pluripotent stem cellsincludes RNA interference knockdown of Gsk3. In other embodiments,inhibiting Gsk3 in the pluripotent stem cells includes overexpression ofa dominant negative form of Gsk3.

In yet other embodiments, the Wnt/β-catenin pathway signaling isactivated in the cultured pluripotent stem cells by overexpressingβ-catenin in the cultured pluripotent stem cells.

In some embodiments of the above-mentioned method, inhibitingWnt/β-catenin signaling in the first cell population includes contactingthe first cell population with a small molecule that inhibits Wnt/βcatenin signaling. In some embodiments, the small molecule that inhibitsWnt/β catenin signaling is a small molecule that stabilizes axin andstimulates β-catenin degradation. In one embodiment, the small moleculethat stabilizes axin and stimulates β-catenin degradation includes3,5,7,8-Tetrahydro-2-[4-(trifluoromethyl)phenyl]-4H-thiopyrano[4,3-d]pyrimidin-4-one(“XAV939”).

In other embodiments, the small molecule that inhibits Wnt/β cateninsignaling is a small molecule that prevents palmitylation of Wntproteins by porcupine. In some embodiments, the small molecule thatprevents palmitylation of Wnt proteins by porcupine includesN-(6-Methyl-2-benzothiazolyl)-2-[(3,4,6,7-tetrahydro-4-oxo-3-phenylthieno[3,2-d]pyrimidin-2-yl)thio]-acetamide(“IWP2”),2-(3,4,6,7-tetrahydro-3-(2-methoxyphenyl)-4-oxothieno[3,2-d]pyrimidin-2-ylthio)-N-(6-methylbenzo[d]thiazol-2-yl)acetamide(“IWP4”), or a combination thereof.

In yet another embodiment, the small molecule that inhibits Wnt/βcatenin signaling is a small molecule that increases the activity levelof casein kinase 1α. In some embodiments, the small molecule thatincreases the activity level of casein kinase 1α is6-(Dimethylamino)-2-[2-(2,5-dimethyl-1-phenyl-1H-pyrrol-3-yl)ethenyl]-1-methyl-4,4′-methylenebis[3-hydroxy-2-naphthalenecarboxylate](2:1)-quinolinium, or a combination thereof.

In further embodiments, inhibiting the Wnt/β-catenin signaling in thefirst cell population includes contacting the first cell population withat least one antibody that blocks activation of a Wnt ligand receptor.In some embodiments, the at least one antibody binds to one or more Wntligand family members. In other embodiments, the at least one antibodybinds to the Wnt ligand receptor.

In other embodiments, inhibiting the Wnt/β-catenin signaling in thefirst cell population comprises reducing β-catenin expression in thefirst cell population. In some embodiments, reducing β-cateninexpression comprises expressing shRNA for β-catenin in the first cellpopulation. In some embodiments, reducing β-catenin expression comprisesoverexpressing Axin2 in the first cell population.

In further embodiments of the above-mentioned method, the pluripotentstem cells in step (i), the first cell population in step (ii), or thesecond population in step (iii) are cultured under exogenous growthfactor-free conditions.

In some embodiments, the second cell population is cultured for a periodafter ending the inhibition of Wnt/β-catenin signaling initiated in step(iii) to obtain a cell population comprising cardiomyocytes.

In some embodiments, where a cell population comprising cardiomyocytesis obtained, at least 70% of the cells in the cell population arecardiac troponinT (cTnT)-positive, and the cell population comprisingthe at least 70% cTnT-positive cells is obtained without the use of acell separation step on the second cell population.

In another aspect, provided herein is a method for culturing pluripotentstem cells to obtain a population of cardiomyocytes, the methodcomprising the steps of: sequentially inhibiting Gsk3 in the pluripotentcells and then inhibiting Wnt signaling in the Gsk3 inhibited cells; andculturing the sequentially inhibited cells in a culture medium to form adifferentiated cell population comprising cardiomyocytes.

These and other features, objects, and advantages of the presentinvention will become better understood from the description thatfollows. In the description, reference is made to the accompanyingdrawings, which form a part hereof and in which there is shown by way ofillustration, not limitation, embodiments of the invention. Thedescription of preferred embodiments is not intended to limit theinvention to cover all modifications, equivalents and alternatives.Reference should therefore be made to the claims recited herein forinterpreting the scope of the invention.

INCORPORATION BY REFERENCE

All publications, patents, and patent applications mentioned in thisspecification are herein incorporated by reference to the same extent asif each individual publication, patent, and patent application wasspecifically and individually indicated to be incorporated by reference.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood and features, aspectsand advantages other than those set forth above will become apparentwhen consideration is given to the following detailed descriptionthereof. Such detailed description makes reference to the followingdrawings, wherein:

FIG. 1A-D illustrate that treatment of hPS cells with Gsk3 inhibitorsenhances cardiac differentiation. FIG. 1A depicts H9-7TGP cells treatedwith different concentrations of CHIR99021(CH) in mTeSR®1 for 4 days.FIG. 1B depicts immunofluorescent staining for Oct4, Isl1, and NRx2.5relative to GFP expression in H9-7TGP cells treated with 12 μM CHIR99021in mTeSR1 for 4 days. Scale bar=50 μm. FIG. 1C depicts H9 cells culturedon MEFs treated with CHIR99021 in hESC medium for 3 days before formingEBs. FIG. 1D depicts percentage of cTnT+ cells in day 15 cultureassessed by flow cytometry. H9 cells were cultured on Matrigel® andtreated with DMSO, 1 μM CHIR99021, or 1 μM BIO for 3 days beforeexposure to 100 ng/ml Activin A at day 0 and 5 ng/ml BMP4 at day 1 inRPMI/B27-insulin medium. #p<0.005, CH versus DMSO, BIO versus DMSO; ttest.

FIG. 2A-E illustrates that treatment of hPS cells with Gsk3 inhibitorsenhances cardiac differentiation. FIG. 2A schematically depicts a 7TGPlentiviral promoter-reporter construct, wherein ‘TCF BS’ represents 7repeats of TCF/LEF consensus promoter binding sites. FIG. 2B depicts GFPexpression detected by immunofluorescent microscopy of H9 7TGP cellscultured in UM on MEF feeders, or in CM, mTeSR1, or mTeSR1+12 μM CH onMatrigel for 3 days following seeding. Scale bar=50 μm. FIG. 2C depictsGFP expression in H1 7TGP cells cultured in mTeSR1+15 μM CH on Matrigelat different time points after CH addition. FIG. 2D depicts GFPlocalization and immunostaining of Oct4 and Isl1 in 19-9-11 7TGP cellscultured in mTeSR1+12 μM CH on Matrigel for 3 days. Scale bar=50 μm.FIG. 2E depicts cTnT expression 15 days after initiation ofdifferentiation in IMR90C4 cells cultured in mTeSR1 containing differentconcentrations of BIO. Differentiation was induced by 100 ng/ml ActivinA at day 0 and 5 ng/ml BMP4 at day 1 in RPMI/B27-insulin medium.#p<0.005, each point versus no BIO; t test.

FIG. 3A-D illustrate that differentiation induced by treatment with Gsk3inhibitors is β-catenin dependent. FIG. 3A schematically depicts aconstruct for constitutive knockdown of β-catenin expression and shRNAsequences targeting β-catenin. FIG. 3B depicts quantitative RT-PCR geneexpression analysis of β-catenin and β-actin for scramble and β-cateninknockdown lines in 19-9-11 and H9 cells. #p<0.005, shcat-1 versusscramble, shcat-2 versus scramble; t test. FIG. 3C depicts RT-PCRanalysis of pluripotent, mesendoderm, early mesoderm, and early cardiacgene expression of same cells. 19-9-11 shcat-2 and scramble cells werecultured on Matrigel with mTeSR1 containing 12 μM CH for 4 days. FIG. 3Ddepicts Oct4 expression measured by flow cytometry of 19-9-11 shcat-2and scramble lines cultured on Matrigel with mTeSR1 containing 12 μM CHfor 4 days. Each line represents an independent replicate. FIG. 3Edepicts relative expression of Tin scramble and shcat-2 lines quantifiedby qPCR. 19-9-11 shcat-2 and scramble lines were cultured on Matrigel inmTeSR1 containing CHIR99021 for 2 days. FIG. 3F depicts flow cytometryanalysis of brachyury expression in 19-9-11 shcat-2 and scramble cellsexposed to CH for 4 days. Error bars represent s.e.m. of 3 independentreplicates. FIG. 3G depicts Nanog and Isl1 expression in 19-9-11 shcat-2and scramble line cultured on Matrigel in mTeSR1 containing 12 μM CH.Scale bar=50 μm.

FIG. 4A-G illustrates temporal modulation of Wnt/β-catenin signalingpromoting cardiac differentiation. FIG. 4A depicts β-catenin expression,measured by RT-PCR, in H9 β-catenin knockdown and scramble cell linescultured in mTeSR1. FIG. 4B depicts β-catenin expression measured byqRT-PCR in H7 shcat-2 and scramble lines. Error bars represent thes.e.m. of three samples. #p<0.005, shcat-2 versus scramble; t test. FIG.4C depicts immunostaining of Oct4 in H9 scramble, shcat-1, shcat-2 cellscultured in mTeSR1 on Matrigel for 3 days. Scale bar=50 μm. FIG. 4Ddepicts Oct4 expression, measured by flow cytometry, in 19-9-11 shcat-2and scramble lines cultured in mTeSR1 on Matrigel for 3 days. The blue(scramble) and green (shcat-2) histogram represents Oct4 expression andthe red histogram is an isotype control. FIG. 4E depicts cellmorphology, which was assessed by phase contrast imaging of 19-9-11shcat-2 and scramble cells cultured on Matrigel in mTeSR1 containing 12μM CH for 4 days. Scale bar=50 μm. FIG. 4F depicts 19-9-11 iscramblecells and FIG. 4G depicts 19-9-11 ishcat-2 cells, each cultured inmTeSR1 for 5 days then exposed to 100 ng/ml Activin A at day 0 and 5ng/ml BMP4 at day 1, with 2 μg/ml doxycycline (dox) addition at theindicated times. 15 days after initiation of differentiation, cells werecounted and analyzed for cTnT expression by flow cytometry. Error barsrepresent the s.e.m. of three independent experiments. #p<0.005, eachtime point versus no dox; t test.

FIG. 5A-D illustrates temporal inhibition of Wnt/β-catenin signalingpromoting cardiac differentiation. FIG. 5A schematically depicts aninducible shRNA construct for β-catenin knockdown and shRNA sequencestargeting β-catenin. FIG. 5B depicts representative phase contrast andmCherry epifluorescence images of 19-9-11 cells transduced withlentvirial vectors containing the constructs described in FIG. 5A andselected by puromyocin treatment. FIG. 5C depicts β-catenin expression,measured by qPCR, in 19-9-11 ishcat-1 and ishcat-2 cells cultured inmTeSR1 containing 2 μg/ml dox for 3 days. Error bars represent thes.e.m. of 3 samples. #p<0.005, ishcat-1 versus iscramble, ishcat-2versus iscramble; t test. FIG. 5D depicts cTnT expression 15 days afterinitiation of differentiation in 19-9-11 ishcat-1 cells cultured inmTeSR1 for 5 days then exposed to 100 ng/ml Activin A at day 0 and 5ng/ml BMP4 at day 1, with 2 μg/ml dox addition at the indicated times.Error bars represent the s.e.m. of three independent experiments.#p<0.005, each time point versus no dox; t test.

FIG. 6A-F illustrates that manipulation of Gsk3 and Wnt signaling issufficient for efficient and reproducible generation of functionalcardiomyocytes in the absence of growth factors. FIG. 6A schematicallydepicts a protocol for defined, growth factor free differentiation ofhPS cells expressing dox-inducible β-catenin shRNA to cardiomyocytes viatreatment with small molecules. FIG. 6B depicts 19-9-11 ishcat-2 cellscultured as indicated in FIG. 6A, with dox added at different timepoints following 12 μM CH treatment. At day 15 cells were analyzed forcTnT expression by flow cytometry. Error bars represent the s.e.m. ofthree independent experiments. *p<0.05; #p<0.005, each time point versusno dox; t test. FIG. 6C depicts cardiomyocytes generated from 19-9-11ishcat-1 cells using the protocol described in FIG. 6A, with 12 μM CHtreatment at day 0 and 2 μg/ml dox treatment 36 hr later. At day 30,cells were individualized and replated on 0.1% gelatin coatedcoverslips. Immunostaining for α-actinin and MLC2a shows sarcomereorganization. Scale bar=50 μm. FIG. 6D depicts transmission electronmicroscopic images of beating clusters derived from 19-9-11 ishcat-1line as described in FIG. 6C. Myofibrils (red arrowhead) with Z-bands(green arrowhead) and mitochondria (blue arrowhead) are shown. Scalebar=2 μm. FIG. 6E depicts microelectrode recordings of action potentialactivity collected at day 29 in cardiomyocytes derived from the 19-9-11ishcat-1 cells differentiated as described in FIG. 6C. Dashed linesindicate 0 mV. FIG. 6F depicts representative recordings of APscollected during field stimulation at 1, 2, and 3 Hz as indicated (top).FIG. 6F also depicts bar graphs showing average (±s.e.m.) fractionalchanges in action potential duration at 90 and 50 percent repolarizationobtained by normalizing to the values observed in response to 1 Hzstimulation (bottom). Data represent s.e.m. of 4 independentexperiments.

FIG. 7A-I illustrates cardiomyocyte content of differentiated cellpopulation. FIG. 7A depicts results of flow cytometry of cTnT performedat day 15 post-addition of CH. 19-9-11 ishcat-1 cells were treated withdifferent concentrations of CH in RPMI/B27-insulin for 24 hr and thenthe medium was changed to RPMI/B27-insulin at day 1. Starting from day7, cells were cultured in RPMI/B27. Error bars represent the S.E.M. ofthree independent experiments. *p<0.05; #p<0.005, each point versus noCH; t test. FIG. 7B depicts cell counts and results of flow cytometry ofcTnT performed at day 15 post-addition of CH. 19-9-11 ishcat-1 cellswere cultured in mTeSR1 before exposure to 12 μM CH in RPMI/B27-insulinfor 24 hr. 2 μg/ml dox was added at different time points following CHaddition. Error bars represent the S.E.M. of three independentexperiments. FIG. 7C depicts results of flow cytometry of cTnTexpression cells performed 15 days following CH addition. 19-9-11ishcat-2 and three additional hPS cell lines (IMR90C4, 6-9-9, and H9)transduced with inducible β-catenin shRNA construct ishcat-1 cells werecultured in mTeSR1 and treated with 12 μM CH followed by 2 μg/ml doxaddition 36 hr later. FIG. 7D-G depict immunostaining of day 30cardiomyocytes generated from (7D) 19-9-11 ishcat-1, (7E) IMR90C4ishcat-1, (7F) 6-9-9 ishcat-1, and (7G) H9 ishcat-1 cells, each culturedin mTeSR1, and treated with 12 μM CH and 2 μg/ml dox addition 36 hrlater. Cells were immunostained for cTnT, α-actinin, and MLC2a to showsarcomere structure. Scale bar=20 μm. FIG. 7H-I depict transmissionelectron microscopy images of beating clusters derived from 19-9-11ishcat-1 cells following culture in mTeSR1 and treatment with 12 μM CHand 2 μg/ml dox addition at 36 hr later. Myofibrils (red arrowhead) withZ-bands (green arrowhead) and intercalated disks with desmosomes (pinkarrowhead) are shown. Scale bar=200 nm.

FIG. 8A-D illustrates molecular characterization of cardiomyocytegenerated via Wnt pathway modulation. FIG. 8A depicts pluripotent,mesendoderm, mesoderm, and cardiac gene expression in 19-9-11 ishcat-1cells differentiated as described in FIG. 6A. At different time points,mRNA was collected and RT-PCR analysis was performed. FIG. 8B-D depictcTNT, MLC2v, MLC2a, and SMA expression, MF20 staining, Ki67 staining,and BrdU incorporation in 19-9-11 ishcat-1 cells differentiated as shownin FIG. 6C, with 12 μM CH added at day 0 and 2 μg/ml doxycline added 36hr later. Error bars represent the s.e.m. of three independentexperiments. Day 20, day 40 and day 60 are significantly different fromeach other (p<0.05) when data were compared using 1-way ANOVA and Tukeypost tests.

FIG. 9A-C illustrates induction of TGFβ superfamily signaling by GSK3inhibitor treatment. FIG. 9A depicts the percentage of cTnT+ cells atday 15 in 19-9-11 ishcat-2 cells treated with 12 μM CH, 12 μM CH and0.5-4 μM SB431542, or 12 μM CH and 0.2-1 μM DMH1 for 24 hr, wherein allsamples were treated with 2 μg/ml dox 48 hr later. Error bars representthe s.e.m. of three independent experiments. *p<0.05; #p<0.005, eachpoint versus control; t test. FIG. 9B depicts expression of BMP2/4 andexpression and phosphorylation of SMAD proteins in 19-9-11 ishcat-2cells undergoing differentiation by 100 ng/ml Activin A at day 0 and 5ng/ml BMP4 at day 1 or treatment with either 12 μM CH, 12 μM CH and 1 μMDMH1, or 12 μM CH and 1 μM SB431542 for 24 hr followed by 2 μg/ml doxaddition at 36 hr. FIG. 9C depicts Wnt and TGFβ pathway genes expressionassessed by RT-PCR in 19-9-11 ishcat-2 cells differentiated tocardiomyocytes as shown in 4A, with 12 μM CH treatment at day 0 and 2μg/ml dox addition at 36 hr.

FIG. 10A-C illustrates protein expression patterns in differentiatedcardiomyocytes. FIG. 10A depicts expression and phosphorylation of Smadproteins in 19-9-11 ishcat-1 cells cultured in mTeSR1 treated with 12 μMCH, 12 μM CH and 0.5-4 μM SB431542, or 12 μM CH and 0.2-1 μM DMH1 for 24hr in RPMI/B27-insulin, at different time points following CH treatment.All samples were treated with 2 μg/ml dox 36 hr later. The plot showsdensitometry measurements of pSmad1/5 protein bands relative to totalSmad1 and pSmad2 protein bands relative to total Smad2. FIG. 10B depictsMF20 expression in day 15 19-9-11 cells cultured in mTeSR1 on Matrigeland treated with 12 μM CH followed by 1 μM IWP4 addition 2 or 3 dayslater. Error bars represent the S.E.M. of three independent experiments.FIG. 10C depicts cTnT expression in day 15 19-9-11 cells cultured inmTeSR1 on Matrigel for 5 days before exposure to indicatedconcentrations of CH98014, BIO-acetoxime, or BIO at day 0 for 24 hr andIWP4 added at day 3, in RPMI/B27-insulin.

FIG. 11A-B illustrates development of a protocol for differentiation ofhPS cells to cardiomyocytes in fully defined conditions via smallmolecule modulation of Gsk3 and Wnt signaling. FIG. 11A depicts cTnTMF20 expression in day 15 19-9-11 cells cultured on Matrigel in mTeSR1for 5 days before exposure to 12 μM CH at day 0 and 0-7 μM IWP4 or 5 μMIWP2 at day 3 in RPMI/B27-insulin. Error bars represent s.e.m. of 3independent experiments. #p<0.005, each point versus no drug; t test.FIG. 11B depicts cTnT MF20 expression in day 15 IMR90C4 and 19-9-11cells cultured on Synthemax® plates in mTeSR1 for 5 days before exposureto 12 μM CH at day 0 and 50 μM IWP4 at day 3 in RPMI/B27-insulin.IMR90C4 cells were differentiated with 100 ng/ml Activin A at day 0 and5 ng/ml BMP4 at day 1 as a control. Error bars represent s.e.m. of 3independent experiments.

While the present invention is susceptible to various modifications andalternative forms, exemplary embodiments thereof are shown by way ofexample in the drawings and are herein described in detail. It should beunderstood, however, that the description of exemplary embodiments isnot intended to limit the invention to the particular forms disclosed,but on the contrary, the intention is to cover all modifications,equivalents and alternatives falling within the spirit and scope of theinvention as defined by the appended claims.

DETAILED DESCRIPTION

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which the invention pertains. Although any methods andmaterials similar to or equivalent to those described herein can be usedin the practice or testing of the present invention, preferred methodsand materials are described herein.

In describing the embodiments and claiming the invention, the followingterminology will be used in accordance with the definitions set outbelow.

As used herein, “about” means within 5% of a stated concentration rangeor within 5% of a stated time frame.

The terms “defined culture medium,” “defined medium,” and the like, asused herein, indicate that the identity and quantity of each mediumingredient is known.

As used herein, the terms “chemically-defined culture conditions,”“fully defined, growth factor free culture conditions,” and“fully-defined conditions” indicate that the identity and quantity ofeach medium ingredient is known and the identity and quantity ofsupportive surface is known.

As used herein, “effective amount” means an amount of an agentsufficient to evoke a specified cellular effect according to the presentinvention.

As used herein, the term “Gsk 3 inhibited cells” refers to (i) cells inwhich Gsk3 has previously been inhibited, but in which Gsk3 is no longeractively inhibited; or (ii) cells in which Gsk3 is not being activelyinhibited, but for which a parental stem cell or progenitor cellpopulation had had Gsk3 inhibited. Within the context of the presentdisclosure such “Gsk 3 inhibited cells” correspond to some embodimentsin which a first cell population (comprising mesendodermal markers) isobtained after exposing human pluripotent stem cells to a Gsk3 inhibitorfor a defined period of time, after which Gsk-3 is no longer activelyinhibited.

As used herein, the term “pluripotent cell” means a cell capable ofdifferentiating into cells of all three germ layers. Examples ofpluripotent cells include embryonic stem cells and induced pluripotentstem (iPS) cells. As used herein, “iPS cells” refer to cells that aresubstantially genetically identical to their respective differentiatedsomatic cell of origin and display characteristics similar to higherpotency cells, such as ES cells, as described herein. The cells can beobtained by reprogramming non-pluripotent (e.g., multipotent or somatic)cells.

As used herein, “iPS cell derivation” means reprogramming a somatic cellto become pluripotent.

The present invention involves a method for differentiating hPS cells toobtain a population of cardiomyocyte progenitors, the method includingthe steps of: sequentially activating Wnt/β-catenin pathway signaling inpluripotent stem cells to obtain a first cell population characterizedby a majority of cells expressing mesodermal or endodermal markers(“mesendoderm” markers). Subsequently, the first population of cells iscultured for a period without further activation of Wnt/β-cateninpathway signaling. Afterwards, Wnt/β-catenin pathway signaling in thefirst population of cells is inhibited for a period of time, and thenrelieved of Wnt/β-catenin pathway signaling to differentiate the firstpopulation of cells into a second cell population containingcardiomyocyte progenitors. In some embodiments, the second cellpopulation, comprising cardiomyocyte progenitors, is then furthercultured for a period of time to obtain a population of cells comprisingcardiomyocytes.

The methods have valuable applications such as inexpensive andreproducible generation of human cardiomyocyte progenitors orcardiomyocytes. Generating cardiomyocyte progenitors or cardiomyocytesin completely chemically-defined conditions might facilitate translationof these cells to regenerative therapies.

As disclosed herein, in some embodiments of the differentiation methods,exogenous TGFβ superfamily growth factors are not required to generatecardiomyocyte progenitors or cardiomyocytes from pluripotent cells.While not wishing to be bound by theory, it is believed that, in variousembodiments described herein, undifferentiated pluripotent stem cells ormesendoderm cells, including those differentiated from pluripotent stemcells, provide sufficient endogenous Nodal and BMP proteins. As shownherein, in some cases, substantially growth factor-free directeddifferentiation, including temporal modulation of Wnt pathway regulatorsas set forth herein, can generate up to 95% cTnT+ cardiomyocytes frompluripotent stem cells.

Another advantage is that in some embodiments of the disclosed methods,differentiation of hPS cells into a differentiated population of cellscomprising cardiomyocyte progenitors or cardiomyocytes is carried outunder chemically-defined conditions, whereas most, if not all, existingprotocols require expression of transcription factors, integration ofcardiac specific promoter driven selection cassettes, or application ofserum and/or growth factors.

As described in further detail below, the inventors' simplifiedprotocols target key regulatory elements of the Wnt/β-catenin signalingpathway, simplifying the steps and components involved in derivingcardiomyocyte progenitors and cardiomyocytes from pluripotent stemcells.

Timing

In some embodiments, in the first step, i.e., step (i) of thejust-mentioned method, pluripotent stem cells to be differentiated aresubjected to activation of Wnt/β-catenin pathway signaling for a periodof about 8 hours to about 48 hours, e.g., about 8 hours, 12 hours, 16hours, 20 hours, 24 hours, 28 hours, 32 hours, 36 hours, 40 hours, 44hours, 48 hours, or another period of Wnt/β-catenin pathway signalingactivation from about 8 hours to about 48 hours to obtain a first cellpopulation of cells, characterized by the expression of mesendodermalmarkers. In one embodiment, pluripotent stem cells are subjected toWnt/β-catenin pathway signaling activation for a period of about 24hours.

In some embodiments, in step (ii) after the end of the Wnt/β-cateninpathway activation step, i.e., after the agent for activating theWnt/β-catenin pathway signaling has been removed or has ended, the firstpopulation of cells is cultured in the absence of external Wnt/β-cateninpathway activation for a period of at least about 8 hours to about 60hours, e.g., about 10 hours, 11 hours, 12 hours, 14 hours, 16 hours, 20hours, 24 hours, 30 hours, 36 hours, 40 hours, 44 hours, 48 hours, 52hours, 56 hours, 60 hours or another period from at least about 8 hoursto about 60 hours. In one embodiment, this culture period is about 12hours.

In one embodiment, this culture period is about 12 hours. In someembodiments, the culture period is about 48 hours.

In some embodiments, in step (iii), immediately after culture of thefirst population of cells in the absence of Wnt/β-catenin pathwayactivation, the first population is subjected to inhibition ofWnt/β-catenin pathway signaling. In some embodiments, step (iii) isinitiated at least about 33 hours to about 74 hours following thebeginning of step (i), e.g., at least about 34 hours, 36 hours, 38hours, 39 hours, 40 hours, 45 hours, 50 hours, 60 hours, 65 hours, 68hours, 69 hours, 70 hours, 72 hours, or another time point from at leastabout 33 hours to about 74 hours following the beginning of step (i). Insome embodiments, step (iii) is initiated 36 hours after the beginningof step (i). In other embodiments, step (iii) begins 72 hours after thebeginning of step (i).

In some embodiments, the medium from step (ii) is only partiallyreplaced with fresh medium to obtain a “combined medium” in which thefirst cell population is cultured at the beginning of step (iii). Insome embodiments, the proportion of fresh medium in the final culturemedium volume at the beginning of step (iii) ranges from about 30% toabout 70%, e.g., about 35%, 40%, 45%, 50%, 55%, 60%, 64% or anotherproportion from about 30% to about 70%. In some embodiments, theproportion of fresh medium in the final culture volume at the beginningof step (iii) is about 50%. In other embodiments, the medium from step(ii) is completely replaced with fresh medium at the beginning of step(iii).

In one embodiment, where β-catenin RNA interference is to be used toinhibit the Wnt/β-catenin signaling pathway, RNA interference isinitiated about 36 hours following the beginning of Gsk3 inhibition. Inanother embodiment, where small molecule-mediated inhibition of theWnt/β-catenin signaling pathway is to be used, the first cell populationis contacted with the small molecule inhibitor at about 3 days followingGsk3 inhibition.

Typically, inhibition of Wnt/β-catenin signaling in the first populationof cells during step (iii) is maintained for a period of at least about1 day to about 6 days, e.g., about 1 day, 2 days, 2.5 days, 3 days, 3.5days, 4 days, 5 days, or another period of Wnt/β-catenin signalinginhibition from at least about 1.5 days to about 6 days. In someembodiments, where a small molecule inhibitor is used to inhibitWnt/β-catenin signaling, the first cell population is contacted with thesmall molecule for a period of about 2 days, and then culture of thefirst cell population continues in the substantial absence of the smallmolecule inhibitor. In other embodiments, where inducible RNAinterference is used (e.g., with an inducing agent such as Doxycyclineto drive expression of tet-on expression cassette) to knockdownexpression of β-catenin, induction and maintenance of β-catenin is for aperiod of about 3.5 days, after which induction of β-catenin shRNAexpression is terminated, and then culture of the first cell populationcontinues in the substantial absence of the inducing agent.

While, in some cases, cells are cultured continuously from the beginningof step (i) to step (iii) to obtain a population comprisingcardiomyocyte progenitors, in other cases cultured cells are removedfrom a culture substrate and frozen for storage thus allowing for thedifferentiation method to be resumed after thawing cells at a laterdate. For example, in some cases, the first population of cells obtainedafter step (i) is collected and stored frozen in any number of suitablecell cryopreservation media known in the art, and then later thawed andcultured to resume the differentiation method starting at step (ii) andcontinuing to step (iii) in which Wnt//β-catenin pathway signaling isinhibited to drive differentiation into a second cell populationcomprising cardiomyocytes.

In other embodiments, the second population of cells, comprisingcardiomyocyte progenitors, is cryopreserved, and thawed at a later datefor continued culture of the second population in order to obtain apopulation comprising cardiomyocytes. Accordingly, one of ordinary skillin the art will appreciate that, where the differentiation methodsdescribed herein include a cell freezing step, the absolute timeinterval between at least two steps will be different from thecorresponding step interval in embodiments that do not include afreezing step.

Typically, the second cell population obtained by the disclosed methodscomprises a very high proportion of cardiomyocyte progenitors. In someembodiments, the second cell population comprises about 50% to about 99%cardiomyocyte progenitors, e.g., about 52%, 55% 67%, 70%, 72%, 75%, 80%,85%, 90%, 95%, 98%, or another percent of cardiomyocyte progenitors fromabout 50% to about 99% cardiomyocyte progenitors.

In some embodiments, after ending the inhibition of Wnt/β-cateninsignaling initiated during step (iii), as described herein, theresulting second population of cells, comprising cardiomyocyteprogenitors, is cultured for an additional period of time to obtain acell population comprising cardiomyocytes. In some embodiments, theadditional cell culture period for the second cell population rangesfrom at least about 20 days to about 200 days, e.g, about 23 days, 25days, 27 days, 30 days, 35 days, 40 days, 45 days, 55 days, 70 days, 90days, 100 days, 120 days, 150 days, 170 days, 180 days, 190 days, oranother culture period, after ending inhibition of Wnt/β-cateninsignaling, from at least about 20 days to about 200 days following theend of Wnt/β-catenin signaling inhibition. In one embodiment, the secondpopulation of cells is cultured for a period of at least about 25 daysafter ending inhibition of Wnt/β-catenin signaling.

In some embodiments, continued culture of the second population (in theabsence of Wnt/β-catenin signaling inhibition) yields a cell populationpopulation comprising about 50% to about 99% cardiomyocytes, e.g., about52%, 55% 67%, 70%, 72%, 75%, 80%, 85%, 90%, 95%, 98%, or another percentof cardiomyocytes from about 50% to about 99% cardiomyocytes.

In some embodiments, no cell separation step or method is used to obtaina second cell population comprising at least 70% cTnT-positive cells. Inother embodiments, cell separation or enrichment methods, e.g., FACS,MACS, or laser-targeted ablation of non-cardiomyotcyes are used toobtain a second cell population further enriched in cardiomyocytesrelative to the second cell population prior to application of a cellseparation or enrichment method. Cardiomyocytes are identified by thepresence of one or more cardiomyocyte markers (e.g., cTnT expression) orfunctional characteristics (e.g., spontaneous contractility).

Useful gene expression or protein markers for identifying cardiomyocyteprogenitors or cardiomyocytes, include, but are not limited to, SmoothMuscle Actin, Cardiac Troponin T, light meromyosin MF20, sarcomericmyosin, Myosin Light Chain ventricular, Myosin Light Chain Atrial, andalpha-actinin, NKX2.5, TBX5, GATA4, MEF2, and combinations thereof. Suchmarkers can be detected at the mRNA expression level or protein level bystandard methods in the art.

In some embodiments, where cardiomyocytes are to be generated, certaincardiomyocyte functional criteria are also assessed. Such functionalcardiomyocyte criteria include, but are not limited to, spontaneouslycontractility, response to electrical pacing, the presence of organizedcontractile structures, or a combination thereof.

Pluripotent stem cells (PSCs) suitable for the differentiation methodsdisclosed herein include, but are not limited to, human embryonic stemcells (hESCs), human induced pluripotent stem cells (hiPSCs), non-humanprimate embryonic stem cells (nhpESCs), non-human primate inducedpluripotent stem cells (nhpiPSCs).

Activation of Wnt/β-Catenin Signaling

As will be appreciated by those of ordinary skill in the art,Wnt/β-catenin signaling can be activated by modulating the function ofone or more proteins that participate in the Wnt/β-catenin signalingpathway to increase β-catenin expression levels or activity, TCF and LEFexpression levels, or β-catenin/TCF/LEF induced transcriptionalactivity.

In some embodiments, activation of Wnt/β-catenin signaling is achievedby inhibition of Gsk3 phosphotransferase activity or Gsk3 bindinginteractions. While not wishing to be bound by theory, it is believedthat inhibition of Gsk3 phosphorylation of β-catenin will inhibit tonicdegradation of β-catenin and thereby increase β-catenin's level andactivity to drive differentiation of pluripotent stem cells to anendodermal/mesodermal lineage. Gsk3 inhibition can be achieved in avariety of ways including, but not limited to, providing small moleculesthat inhibit Gsk3 phosphotransferase activity, RNA interferenceknockdown of Gsk3, and overexpression of dominant negative form of Gsk3.Dominant negative forms of Gsk3 are known in the art as described, e.g.,in Hagen et al (2002), J Biol Chem, 277(26):23330-23335, which describesa Gsk3 comprising a R96A mutation.

In some embodiments, the Wnt/β-catenin signaling pathway is activated byinhibiting Gsk3 in pluripotent stem cells by contacting the pluripotentstem cells with a small molecule that inhibits Gsk3 phosophotransferaseactivity or Gsk3 binding interactions. Suitable small molecule Gsk3inhibitors include, but are not limited to, CHIR 99021, CHIR 98014,BIO-acetoxime, BIO, LiCl, SB 216763, SB 415286, AR A014418,1-Azakenpaullone, Bis-7-indolylmaleimide, and any combinations thereof.In some embodiments, any of CHIR 99021, CHIR 98014, and BIO-acetoximeare used to inhibit Gsk3 in pluripotent stem cells in thedifferentiation methods described herein. In one embodiment, the smallmolecule Gsk3 inhibitor to be used is CHIR99021 at a concentrationranging from about 5 μM to about 20 μM, e.g., about 6 μM, 8 μM, 10 μM,12 μM, 14 μM, 16 μM, or another concentration of CHIR99021 from about 5μM to about 20 μM. In another embodiment, the small molecule Gsk3inhibitor to be used is CHIR 98014 at a concentration ranging from about0.2 μM to about 2 μM, e.g., about 0.6 μM, 0.8 μM, 1 μM, 1.2 μM, 1.4 μM,1.6 μM, or another concentration of CHIR99021 from about 0.2 μM to about2 μM.

In other embodiments, Gsk3 activity is inhibited by RNA interferenceknockdown of Gsk3. For example, Gsk3 expression levels can beknocked-down using commercially available siRNAs against Gsk3, e.g.,SignalSilence® GSK-3α/β siRNA (catalog #6301 from Cell SignalingTechology®, Danvers, Mass.), or a retroviral vector with an inducibleexpression cassette for Gsk3, e.g., a commercially availableTet-inducible retroviral RNAi system from Clontech (Mountainview,Calif.) Catalog No. 630926, or a cumate-inducible system from SystemsBiosciences, Inc. (Mountainview, Calif.), e.g., the SparQ® system,catalog no. QM200PA-2. In other embodiments, the Wnt/β-catenin signalingpathway is activated by overexpressing β-catenin itself, e.g., humanβ-catenin (GenBank Accession Nos: X87838 and CAA61107.1 for nucleotideand protein sequences, respectively). In one embodiment, β-cateninoverexpression is inducible β-catenin overexpression achieved using,e.g., any of the just-mentioned inducible expression systems.Alternatively, a constitutively active, stabilized isoform of β-cateninis used, which contains point mutations S33A, S37A, T41A, and S45A asdescribed, e.g., in Baba et at (2005), Immunity, 23(6):599-609.

In yet other embodiments, Wnt/β-catenin signaling pathway activation inpluripotent stem cell is achieved by contacting the cells with an agentthat disrupts the interaction of β-catenin with Axin, a member of theβ-catenin destruction complex. Disruption of Axin-β-catenin interactionallows β-catenin to escape degradation though the destruction complexthereby increasing the net level of β-catenin to drive β-cateninsignaling. For example, the Axin-β-catenin interaction can be disruptedin pluripotent cells by contacting them with the compound5-(Furan-2-yl)-N-(3-(1H-imidazol-1-yl)propyl)-1,2-oxazole-3-carboxamide(“SKL2001”), which is commercially available, e.g., as catalog no.681667 from EMD4 Biosciences. An effective concentration of SKL2001 toactivate Wnt/β-Catenin signaling ranges from about 10 μM to about 100 μMabout 20 μM, 30 μM, 40 μM, 50 μM, 60 μM, 70 μM, 80 μM, 90 μM or anotherconcentration of SKL2001 from about 10 μM to about 100 μM.

Inhibition of Wnt/β-Catenin Signaling

Inhibition of Wnt/β-catenin pathway signaling means inhibition ofTCF/LEF-β-catenin mediated gene transcription. Inhibition ofWnt/β-catenin pathway signaling can be achieved in a variety of waysincluding, but not limited to: providing small molecule inhibitors, RNAinterference of, or blocking antibodies against functional canonical Wntligands or Wnt pathway receptors (e.g., Frizzled and LRP5/6); providingsmall molecules that promote degradation of β-catenin and/or TCF/LEF;gene interference knockdown of β-catenin and/or TCF/LEF; overexpressionof a dominant negative form of β-catenin lacking the sequence forbinding to TCF/LEF; overexpressing Axin2 (which increases (β-catenindegradation); providing a small molecule inhibitor of a TCF/LEF andβ-catenin interaction; and providing a small molecule inhibitor of aTCF/LEF-β-catenin and DNA promoter sequence interaction.

In some cases, inhibition of Wnt/β-catenin pathway signaling in a firstcell population comprising cells expressing mesendodermal or mesodermalmarkers is achieved by contacting the first cell population with one ormore small molecule inhibitors of a Wnt ligand (e.g., a small moleculethat inhibit secretion of the Wnt ligand) o or inhibit Wnt ligands andtheir corresponding receptors interaction. Suitable small moleculeinhibitors include, but are not limited to,N-(6-Methyl-2-benzothiazolyl)-2-[(3,4,6,7-tetrahydro-4-oxo-3-phenylthieno[3,2-d]pyrimidin-2-yl)thio]-acetamide(“IWP2”) available commercially, e.g., as Sigma catalog no. IO536;2-(3,4,6,7-tetrahydro-3-(2-methoxyphenyl)-4-oxothieno[3,2-d]pyrimidin-2-ylthio)-N-(6-methylbenzo[d]thiazol-2-yl)acetamide(“IWP4”) available commercially, e.g., as catalog no. 04-00306 fromStemgent (San Diego, Calif.);4-(1,3,3a,4,7,7a-Hexahydro-1,3-dioxo-4,7-methano-2H-isoindol-2-yl)-N-8-quinolinyl-Benzamide(“IWR-1”) available commercially, e.g., as Sigma catalog no. 10161;Benzoic acid, 2-phenoxy-, 2-[(5-methyl-2-furanyl)methylene]hydrazide(“PNU-74654”), e.g., Sigma catalog no. P0052; or a combination thereof.

In some embodiments, the first population of cells is contacted with oneor more small molecule compounds that promote degradation of β-catenin.In some cases, such small molecule compounds are compounds that,directly or indirectly, stabilize Axin, which is a member of theβ-catenin destruction complex, and thereby enhance degradation ofβ-catenin. Examples of Axin-stabilizing compounds include, but are notlimited to,3,5,7,8-Tetrahydro-2-[4-(trifluoromethyl)phenyl]-4H-thiopyrano[4,3-d]pyrimidin-4-one(“XAV939”), e.g., Sigma catalog no. X3004;4-(1,3,3a,4,7,7a-Hexahydro-1,3-dioxo-4,7-methano-2H-isoindol-2-yl)-N-8-quinolinyl-Benzamide(“IWR-1”) available commercially, e.g., as Sigma catalog no. 10161. Insome cases, such small molecule compounds that, directly or indirectly,activate casein kinase 1α (CK1), which is a member of the β-catenindestruction complex, and thereby enhances degradation of β-catenin.Examples of CK1-stabilizing compounds include, but are not limited to,6-(Dimethylamino)-2-[2-(2,5-dimethyl-1-phenyl-1H-pyrrol-3-yl)ethenyl]-1-methyl-4,4′-methylenebis[3-hydroxy-2-naphthalenecarboxylate](2:1)-quinolinium (“Pyrvinium pamoate salt hydrate”), e.g., Sigmacatalog no. P0027.

A suitable working concentration range for such small moleculeinhibitors is from about 0.1 μM to about 100 μM, e.g., about 2 μM, 5 μM,7 μM, 10 μM, 12 μM, 15 μM, 18 μM, or another working concentration ofone or more the foregoing small molecule inhibitors ranging from about0.1 μM to about 100 μM. In one embodiment, IWP2 or IWP4 are used at aworking concentration of about 5 μM. In other embodiments, theabove-mentioned small molecule inhibitors are used at the correspondingtarget IC₅₀.

In other embodiments, inhibition of Wnt/β-catenin pathway signaling inthe first cell population is enabled by RNA interference to decrease theexpression of one or more targets in the Wnt/β-catenin pathway. Forexample in some cases, RNA interference is against β-catenin itself. Inone embodiment, where one or more short hairpin interfering RNAs(shRNAs) are to be used to knock down β-catenin expression, at least oneof the following shRNA sequences are used: (SEQ ID NO:63)5′-CCGGAGGTGCTATCTGTCTGCTCTACTCGAGTAGAGCAGACAGATAGCACCTTTTT T-3′ or (SEQID NO:64) 5′-CCGGGCTTGGAATGAGACTGCTGATCTCGAGATCAGCAGTCTCATTCCAAGCTTTTT-3′. Such shRNAs may be transfected as synthetic shRNAs intothe first cell population by a number of standard methods known in theart. Alternatively, shRNA sequences may be expressed from an expressionvector, e.g., from a plasmid expression vector, a recombinantretrovirus, or a recombinant lentivirus.

In some embodiments, the first cell population is generated from agenetically modified pluripotent stem cell line comprising an inducibleexpression cassette for expression of an interfering RNA, e.g., an shRNAagainst β-catenin, as exemplified herein. The use of an inducibleexpression cassette allows temporal control of β-catenin knockdown. Suchtemporal control is well suited to the timing of Wnt/β-catenin signalinginhibition used in the differentiation methods described herein.

As an alternative method for inhibiting Wnt/β-catenin signaling in thefirst cell population, the first cell population is contacted with atleast one antibody that blocks activation of a Wnt ligand receptor. Insome embodiments, the at least one antibody binds to one or more Wntligand family members and inhibits binding of the one or more Wntligands to their receptors. Such antibodies are known in the art, asdescribed in, e.g. an anti-Wnt-1 antibody described in He et at (2004),Neoplasia, 6(1):7-14. In other embodiments, the blocking antibody istargeted against a Wnt ligand receptor and blocks the interaction of Wntligands with the receptor, as described, e.g., in Gurney et al (2012),Proc. Natl. Acad. Sci. USA, 109(29):11717-11722.

Culture Media

Defined media and substrate conditions for culturing pluripotent stemcells, as used in the methods described herein, are well known in theart. In some exemplary embodiments, pluripotent stem cells to bedifferentiated according to the methods disclosed herein are cultured inmTESR-1® medium (StemCell Technologies, Inc., Vancouver, Calif.), orEssential 8® medium (Life Technologies, Inc.) on a Matrigel substrate(BD Biosciences, NJ) according to the manufacturer's protocol or on aCorning® Synthemax surface.

Upon initiating the first step and throughout the differentiationmethods provided herein, pluripotent cells are typically cultured in amedium substantially free of insulin. In some embodiments, a mediumcomprising the supplement B-27 (minus insulin) (Life Technologies,catalog no. 0050129SA) is used throughout the differentiation process.In one embodiment, the medium used for differentiation method comprisesthe following components: 0.1 mg/ml Apo-transferrin, 30 μM Sodiumselenite, 0.02 μg/ml Progesterone, 16 μg/ml Putrescine, and 50 μg/ml BSA(termed “L5” herein). In some embodiments, the cell culture media usedfor the differentiation methods described herein are substantially freeof Activin. In other embodiments, the medium used in the step (i)includes about 100 ng/ml Activin.

A number of known base culture media are suitable for use throughout thedifferentiation methods described herein. Such cell base cell culturemedia include, but are not limited to, RPMI, DMEM/F12 (1:3), DMEM/F12(1:1), DMEM/F12 (3:1), F12, DMEM, and MEM.

In one embodiment, the cell culture medium used is RPMI supplementedwith B27 (minus insulin). In another embodiment, the cell culture mediumused is RPMI supplemented with L5. In yet another embodiment, the cellculture medium used is DMEM/F12 (1:3) supplemented with B27 (minusinsulin).

The invention will be more fully understood upon consideration of thefollowing non-limiting Examples. It is specifically contemplated thatthe methods disclosed are suited for pluripotent stem cells generally.All papers and patents disclosed herein are hereby incorporated byreference as if set forth in their entirety.

EXAMPLES Example 1 Experimental Procedures

Maintenance of hPS Cells:

Transgene free human iPSCs (6-9-9 and 19-9-11) (Yu et al., Science 324,797-801 (2009)), lentiviral integrated human iPSC (IMR90C4) (Yu et al.,Science 318, 1917-1920 (2007)) and hESCs (H1, H7, H9) (Thomson et al.,Science 282, 1145-1147 (1998)) were maintained on MEF feeders in hESCmedium: DMEM/F12 culture medium supplemented with 20% KnockOut serumreplacer, 0.1 mM non-essential amino acids, 1 mM L-glutamine (all fromInvitrogen), 0.1 mM β-mercaptoethanol (Sigma) and 10 ng/ml human bFGF(Invitrogen). For feeder free culture, hPS cells were maintained onMatrigel (BD Biosciences) or Synthemax plates (Corning) in mTeSR1medium.

Cardiac Differentiation Via EBs:

hPS cells were passaged onto MEFs (˜13,000 cells/cm²) and cultured inhESC medium for 2 days followed by another 3 days in hESC mediumsupplemented with BIO (Sigma) or CHIR99021 (Selleck). To form EBs, hPScell aggregates generated by dispase treatment were cultured inlow-attachment plates overnight in RPMI plus 20% KnockOut serumreplacer. The next day, the EBs were cultured in EB20 (RPMI plus 20%FBS) for 4 days in suspension. EBs were then plated onto 0.1%gelatin-coated 6-well culture plates at 50-100 EBs/well, and cultured inEB20 medium. After 10 days of differentiation, the FBS concentration wasreduced to 2% EB2 (RPMI plus 2% FBS). The number of contracting EBs wasvisually assessed using a microscope with a 37° C. heated stage.

Cardiac Directed Differentiation Using Activin A and BMP4:

hPS cells maintained on Matrigel in mTeSR1 were dissociated into singlecells with Accutase (Invitrogen) at 37° C. for 5 min and then seededonto a Matrigel-coated cell culture dish at 100,000 cell/cm² in mTeSR1supplemented with 5 μM ROCK inhibitor (Y-27632, Stemgent) (day −5).Cells were cultured in mTeSR1, changed daily. At day 0, cells weretreated with 100 ng/ml Activin A (R&D) in RPMI/B27-insulin. After 24 hr,the medium was changed to RPMI/B27-insulin supplemented with 5 ng/mlBMP4 (R&D) for another 4 days. At day 5, the medium was changed withRPMI/B27-insulin. At day 7 the cells were transferred to RPMI/B27, andmedium changed every 3 days. When using GSK3 inhibitors to stimulatedifferentiation, cells were cultured in mTeSR1 containing BIO orCHIR99021 from day −3 to day 0.

Cardiac Directed Differentiation Via Small Molecules:

Cells were dissociated and plated as described in the Activin/BMP4protocol. When hPS cells maintained on Matrigel or Synthemax platesachieved confluence, cells were treated with CH in RPMI/B27-insulin for24 hr (day 0 to day 1). The medium was changed to RPMI/B27-insulin,followed by 2 μg/ml dox treatment at different times between day 1 andday 5 for transgenic cell lines. For genetically unmodified lines, 5 μMIWP2 (Tocris) or IWP4 (Stemgent) was added at day 3 and removed duringthe day 5 medium change. Cells were maintained in the RPMI/B27 startingfrom day 7, with the medium changed every 3 days.

Lentiviral Production and Infection of hPS Cells:

The pLKO.1 based constitutive knockdown of β-catenin vectors shcat-1 andshcat-2 (Addgene plasmids 19761 and 19762) and inducible knockdownβ-catenin vectors ishcat-1 and ishcat-2 (Biosettia) were used forlentivirus particle production. These vectors were cotransfected withthe helper plasmids psPAX2 and pMD2.G (Addgene plasmids 12260 and 12259)into HEK-293TN cells (System Biosciences) for virus production.Virus-containing media were collected at 48 and 72 hr after transfectionand used for hPS cell infection in the presence of 6 μg/ml polybrene(Sigma). Transduced cells were selected and clonally isolated based onresistance to 1 μg/ml puromycin.

RT-PCR and Quantitative RT-PCR:

Total RNA was prepared with the RNeasy mini kit (QIAGEN) and treatedwith DNase (QIAGEN).1 μg RNA was reverse transcribed into cDNA via Oligo(dT) with Superscript III Reverse Transcriptase (Invitrogen). Real-timequantitative PCR was done in triplicate with iQ SYBR Green SuperMix(Bio-Rad). RT-PCR was done with Gotaq Master Mix (Promega) and thensubjected to 2% agarose gel electrophoresis. ACTB was used as anendogenous control. Primer sequences are set forth in Table 1.

TABLE 1 SEQ Genes Sequences (5′-3′) ID NO: Size/Tm/CyclePrimers for RT-PCR and Q-PCR OCT4 F: CAGTGCCCGAAACCCACAC 1 161/58/30R: GGAGACCCAGCAGCCTCAAA 2 NANOG F: CGAAGAATAGCAATGGTGTGACG 3 328/58/30R: TTCCAAAGCAGCCTCCAAGTC 4 SOX2 F: CAAGATGCACAACTCGGAGA 5 300/58/30R: GTTCATGTGCGCGTAACTGT 6 CTNNB1 F: GAATGAGACTGCTGATCTTGGAC 7 250/58/30R: CTGATTGCTGTCACCTGGAG 8 GSC F: CGAGGAGAAAGTGGAGGTCTG G 9 261/55/35R: GCAGCGCGTGTGCAAGAAA 10 MIXL1 F: CAGAGTGGGAAATCCTTCCA 11 231/58/35R: TGAGTCCAGCTTTGAACCAA 12 T F: CTTCCCTGAGACCCAGTTCA 13 289/58/35R: CAGGGTTGGGTACCTGTCAC 14 MSX1 F: CCGAGAGGACCCCGTGGATGC 15 280/58/35R: GCCTCTTGTAGTCTCTTTGCC 16 ISL1 F: CACAAGCGTCTCGGGATT 17 202/58/40R: AGTGGCAAGTCTTCCGACA 18 WNT3A F: GCCCCACTCGGATACTTC T 19 189/58/40R: GGCATGATCTCCACGTAGT 20 WNT8A F: ACAGGTCCCAAGGCCTATCT 21 335/58/40R: ATCCTTTCCCCAAATTCCAC 22 NKX2-5 F: GCGATTATGCAGCGTGCAATGAGT 23220/58/35 R: AACATAAATACGGGTGGGTGCGTG 24 GATA4 F: TCCAAACCAGAAAACGGAAG25 352/58/40 R: AAGACCAGGCTGTTCCAAGA 26 MEF2C F: AGCCCTGAGTCTGAGGACAA 27195/58/40 R: GTGAGCCAGTGGCAATAGGT 28 TBX5 F: GAAACCCAGCATAGGAGCTG 29191/58/40 R: CAGCCTCACATCTTACCCTGT 30 TBX2 F: AGTGGATGGCTAAGCCTGTG 31249/58/40 R: ACGGGTTGTTGTCGATCTTC 32 TNNI3 F: CTGCAGATTGCAAAGCAAGA 33379/58/40 R: CCTCCTTCTTCACCTGCTTG 34 TNNT2 F: TTCACCAAAGATCTGCTCCTCGCT35 165/58/40 R: TTATTACTGGTGTGGAGTGGGTGTGG 36 MYL7F: GAGGAGAATGGCCAGCAGGAA 37 449/58/35 R: GCGAACATCTGCTCCACCTCA 38 MYL2F: ACATCATCACCCACGGAGAAGAGA 39 164/58/40 R: ATTGGAACATGGCCTCTGGATGGA 40PLN F: ACAGCTGCCAAGGCTACCTA 41 191/58/40 R: GCTTTTGACGTGCTTGTTGA 42 CD31F: GCTGACCCTTCTGCTCTGTT 43 238/55/35 R: TGAGAGGTGGTGCTGACATC 44 NODALF: CTTCCTGAGCCAACAAGAGG 45 197/58/40 R: AGGTGACCTGGGACAAAGTG 46 BMP2F: TCAAGCCAAACACAAACAGC 47 197/58/40 R: ACGTCTGAACAATGGCATGA 48 BMP4F: TGAGCCTTTCCAGCAAGTTT 49 180/58/40 R: CTTCCCCGTCTCAGGTATCA 50 NOGGINF: TCGAACACCCAGACCCTATC 51 298/58/40 R: TGTAACTTCCTCCGCAGCTT 52 GAPDHF: CCCCTTCATTGACCTCAACTACA 53 342/58/30 R: TTGCTGATGATCTTGAGGCTGT 54ACTB F: CCTGAACCCTAAGGCCAACCG 55 400/58/30 R: GCTCATAGCTCTTCTCCAGGG 56Primers for quantitative RT-PCR GAPDH F: GTGGACCTGACCTGCCGTCT 57 152R: GGAGGAGTGGGTGTCGCTGT 58 T F: AAGAAGGAAATGCAGCCTCA 59 101R: TACTGCAGGTGTGAGCAAGG 60 CTNNB1 F: CCCACTAATGTCCAGCGTTT 61 217R: AACGCATGATAGCGTGTCTG 62

Flow Cytometry:

Cells were dissociated into single cells and then fixed with 1%paraformaldehyde for 20 min at room temperature and stained with primaryand secondary antibodies in PBS plus 0.1% Triton X-100 and 0.5% BSA. Forthe cell proliferation assay, a 17-hour pulse of BrdU was performed toidentify dividing cardiomyocytes (Zhang et al., Circ. Res. 104, e30-41(2009)). Data were collected on a FACSCaliber flow cytometer (BecktonDickinson) and analyzed using FlowJo. Antibodies are set forth in Table2.

TABLE 2 Antibodies for immunostaining (IS), western blotting (WB) andflow cytometry (FC). Antibody Application Actin, Smooth Lab Vision;mouse IgG2a, Clone: 1A4; Ms-113-p 1:100 (FC) muscle Cardiac Troponin TLab Vision; mouse IgG1; Clone: 13-11; ms-295-p1 1:200 (FC) MF20Developmental Studies Hybridoma Bank; mouse 1:20 (FC) IgG2b MLC2vProteinTech Group; Rabbit polyclonal; PTG10906- 1:200 (FC) 1-AP MLC2aSynaptic systems; mouse IgG2b, Clone: 56F5; Cat: 1:200 (IS and 311011FC) α-actinin Sigma; mouse IgG1; Clone: EA-53 1:500 (IS) Brachyury R&D;Polyclonal Ab, Goat IgG; Clone: AF2085 1:100 (FC) ISL1 DSHB; mouseIgG2b; Clone: 39.4D5-s 1:20 (IS) Oct-3/4 Santa Cruz; Rabbit IgG; Clone:H-134; sc-9081 1:40 (FC) Oct-3/4 Santa Cruz; Mouse IgG_(2b); Clone:C-10; sc-5279 1:100 (IS) NKX2-5 Santa Cruz; Rabbit IgG; Clone: H-114;sc-14033 1:75 (IS) Ki67 BD Bioscience; mouse IgG1; Cat: 550609 1:100(FC) BrdU Invitrogen; mouse IgG1; PRB-1; Cat: A21300; 1:100 (FC) Lot:612217 Phospho-Smad1/5 Cell Signaling Technology; Rabbit mAb; 41D10;1:500 (WB) (Ser463/465) Cat: 9516S Phospho-Smad2 Cell SignalingTechnology; Rabbit mAb; 138D4; 1:500 (WB) (Ser465/467) 3108S Smad1 CellSignaling Technology; Rabbit mAb; D59D7; 1:1000 (WB) 6944P Smad2/3 CellSignaling Technology; Rabbit IgG; Cat: 5678S 1:1000 (WB) BMP2/4 SantaCruz; mouse IgG2a; Clone H-1; sc-137087 1:500 (WB) β-Actin CellSignaling Technology; Rabbit mAb (HRP 1:1000 (WB) Conjugate); 13E5;5125S goat anti-mouse IgG- Santa Cruz, sc-2005 1:1000 (WB) HRP

Immunostaining:

Cells were fixed with 4% paraformaldehyde for 15 min at room temperatureand then stained with primary and secondary antibodies in PBS plus 0.4%Triton X-100 and 5% non-fat dry milk (Bio-Rad). Nuclei were stained withGold Anti-fade Reagent with DAPI (Invitrogen). An epifluorescencemicroscope (Leica DM IRB) with a QImaging® Retiga 4000R camera was usedfor imaging analysis. Antibodies are set forth in Table 2.

Western Blot Analysis:

Cells were lysed in M-PER Mammalian Protein Extraction Reagent (Pierce)in the presence of Halt Protease and Phosphatase Inhibitor Cocktail(Pierce). Proteins were separated by 10% Tris-Glycine SDS/PAGE(Invitrogen) under denaturing conditions and transferred to anitrocellulose membrane. After blocking with 5% milk in TBST, themembrane was incubated with primary antibody overnight at 4° C. Themembrane was then washed, incubated with an anti-mouse/rabbitperoxidase-conjugated secondary antibody (1:1000, Cell Signaling) atroom temperature for 1 hr, and developed by SuperSignalchemiluminescence (Pierce). Antibodies are listed in Table 2.

Electrophysiology:

Beating cardiomyocyte clusters were microdissected and replated ontoglass coverslips and maintained in EB2 medium prior to recording. Actionpotential activity was assessed using glass microelectrodes (50-100 MΩ;3 M KCl) in a 37° C. bath continuously perfused with Tyrode's solution(mmole/L): 140 NaCl, 5.4 KCl, 1.8 CaCl₂, 1 MgCl₂, 10 Hepes, 10 glucose,pH 7.4 NaOH). Junction potentials and capacitance were nulled and dataacquired at 10 kHz with an AxoClamp2A amplifier and pClamp 9.2 software(Molecular Devices, Sunnyvale Calif.). Electrical field stimulation wasperformed using two platinum electrodes coupled to a Grass SD-9stimulator (Quincy, Mass.). For analysis, data were filtered off-lineusing a low pass Gaussian filter with a cut-off frequency of 2 kHz.

Electron Microscopy:

Contracting areas were microdissected and re-plated onto gelatin coatedglass coverslips. The clusters were fixed overnight at 4° C. in a 2.5%gluteraldehyde, 2% paraformaldehyde, 0.1 M cacodylate buffer solutionand then post-fixed with 1% osmium tetroxide. Samples were dehydratedvia an ethanol gradient and embedded in durcapan (Fluka). The glasscoverslip was dissolved with hydrofluoric acid treatment. Ultrathin 60nm sections were stained with uranyl acetate and lead citrate. Sampleswere visualized on a Phillips CM120 STEM.

Statistics:

Data are presented as mean±standard error of the mean (SEM). Statisticalsignificance was determined by Student's t-test (two-tail) for twogroups or one-way ANOVA for multiple groups with post hoc testing usingTukey method using Microcal Origin, v8.0. P<0.05 was consideredstatistically significant.

Example 2 Wnt/β-Catenin Pathway Activation by Gsk3 Inhibitors AbrogateshPS Cell Self-Renewal and Enhances Cardiac Differentiation

In order to probe the activation of canonical Wnt/β-catenin signalingduring hPS cell specification to cardiomyocytes, the inventors generateda series of promoter-reporter cell lines in H1 and H9 hESC, and 19-9-11human iPSC lines. These reporter lines, integrated with a lentiviral7TGP vector, express GFP under control of a consensus TCF/LEF bindingsequence promoter which reports β-catenin activation (FIG. 2A) (Fuererand Nusse, PLoS One 5:e9370 (2010)). Whereas Wnt/β-catenin activationhas been reported in undifferentiated mESCs and hESCs (Anton et al.,FEBS Lett. 581:5247-5254 (2007); Sato et al., Nat. Med. 10:55-63 (2004);Takao et al., Biochem. Bioph. Res. Co. 353:699-705 (2007)), theinventors and others (Dravid et al., Stem Cells 23: 1489-1501 (2005))failed to observe significant TCF/LEF-mediated transcriptional activityin self-renewing H9 hESCs in several different culture conditions,including mouse embryonic fibroblast (MEF) co-culture, in MEFconditioned medium (CM) on Matrigel, and in mTeSR1 medium on Matrigel(FIG. 2B). However, treatment of the H9-7TGP reporter line with the GSK3inhibitor CHIR99021 (CH) (Ying et al., Nature 453:519-U515 (2008))activated TCF/LEF promoter activity in a CH concentration-dependentmanner (FIG. 1A; data represent the mean±SEM of 3 experiments).Approximately 50% of H9-7TGP cells expressed GFP at 4 days post-additionof 15 μM CH. Similar results were obtained following CH treatment of WNTreporter line H1-7TGP (FIG. 2C). Immunofluorescent analysis revealedthat the CH-induced H9-7TGP GFP+ cells did not express Oct4, but didexpress Isl1 and Nkx2.5 (FIG. 1B). Similar results were obtained in the19-9-11 7TGP iPSC line (FIG. 2D), suggesting commitment of this GFP+cell population to the early cardiomyocyte lineage.

Since GSK3 inhibition did not support pluripotency of hPS cells butinstead induced differentiation toward cardiac lineages, the inventorsquantitatively assessed the effect of incorporating GSK3 inhibitorsduring previously reported embryoid body (EB) and modified Activin A/BMPdirected differentiation protocols (Laflamme et al., Nat. Biotechnol. 25(9):1015 (2007)). Undifferentiated H9 hESCs were cultured in thepresence of 0-8 μM CH for three days prior to EB formation. EBs werecultured in suspension for 4 days before transferring to 0.1% gelatincoated plates. Visual analysis of spontaneously-contracting outgrowthsindicated that the efficiency of cardiomyocyte differentiation peaked at2-4 μM CH (FIG. 1C). The inventors also applied the GSK3 inhibitors CHand BOP to undifferentiated H9 hESCs three days prior to directeddifferentiation to cardiomyocytes via Activin A and BMP4. This protocolgenerated very few contracting cardiomyocytes in H9 cells. However,application of GSK3 inhibitors CH or BIO 3 days prior to addition ofgrowth factors greatly enhanced cardiomyocyte generation, yielding anaverage of 50% cTNT-labeled cells (FIG. 1D; p<0.005, CH versus DMSO, BIOversus DMSO; t test). Three days of BIO pretreatment prior to additionof Activin A and BMP4 also enhanced generation of cTNT-expressing cellsin the IMR90C4 iPSC line in a dose-dependent manner (FIG. 2E).

Example 3 Differentiation Induced by Gsk3 Inhibitor Treatment isβ-Catenin Dependent

In order to more fully evaluate the role of canonical Wnt signaling incardiomyocyte specification of hPS cells, the inventors generated H7, H9and 19-9-11 iPSC clones carrying lentiviral integrated β-catenin shRNAand control scrambled sequences (FIG. 3A). Referring to FIG. 2A, P_(U6)and P_(hPGK) are human U6 and human PGK promoters, red and greensequences are forward and reverse shRNA sequence of β-catenin, and theblue sequence represents the loop sequence. The β-catenin knockdownefficiency of the shRNA knockdown lines compared to the scrambled lineswas 77% for β-catenin shRNA sequence 1 (shcat-1) and 96% for sequence 2(shcat-2) (FIGS. 3B, 4A and 4B). Knockdown of β-catenin did not abrogatehESC or iPSC self-renewal, as shown by Oct4 expression (FIGS. 4C and4D).

To test whether induction of cardiornyocyte differentiation by GSK3inhibitors requires β-catenin, the inventors treated the shcat-2 andscramble 19-9-11 lines with CH in mTeSR1 medium. While the shcat-2 linemaintained an undifferentiated morphology, the scramble line appeared toundergo differentiation (FIG. 2E), similar to non-transduced linestreated with CH (FIGS. 1B and 2B). Induction of differentiationfollowing CH treatment in the scramble control was further indicated bydiminished SOX2 and NANOG expression at day 4 (FIG. 3C). No significantdifferences in expression of these genes were detected in shcat-2 cellstreated with CH. After 4 days of CH treatment in mTeSR1, the percentageof cells expressing Oct4 decreased to 61% in the scramble line, while98% of the shcat-2 cells expressed Oct4 (FIG. 3D). In addition,expression of mesendoderm and early mesoderm genes MIXL1 GSC, T, WNT3Aand MSX1 emerged in CH-treated scramble cells but not CH-treated shcat-2cells (FIG. 3C). To better understand early mesoderm induction via GSK3inhibition, the inventors analyzed expression of the early mesoderm geneT in scramble and shcat-2 19-9-11 lines. As CH concentration increased,the ratio of T expression in scramble to the shcat-2 line increased(FIG. 3E). Less than 2% of shcat-2 cells expressed brachyury uponexposure to different concentrations of CH treatment for 4 days. Incontrast, the scramble line exhibited a CH concentration-dependentincrease in the fraction of cells expressing brachyury, with 76%brachyury+ cells following 15 μM CH treatment for 4 days (FIG. 3F). Inaddition, immunostaining of the scramble cell line after 15 μM CHtreatment in mTeSR1 for 4 days showed substantial numbers ofNanog-/Isl1+ cells while the shcat-2 cells treated with CH containedonly Nanog+/Isl1-cells (FIG. 3G). Together these results demonstratethat treatment of undifferentiated hPS cells with GSK3 inhibitorsinduces mesoderm differentiation in a β-catenin dependent manner andthat these mesodermal cells can differentiate to Isl1+ and Nkx2.5+ (FIG.1B) cardiomyocyte progenitors.

Example 4 Temporal Requirement of β-Catenin for Efficient CardiacDifferentiation

Building on the finding that β-catenin is a key mediator ofcardiomyocyte induction by Gsk3 inhibitors, the inventors generated19-9-11 iPSC lines, ishcat-1 and ishcat-2, expressing two differentβ-catenin shRNA sequences under control of a Tet-regulated induciblepromoter (FIG. 5A). Referring to FIG. 5A, P_(H1TetO) represents thehuman H1 promoter with Tet operator sequences, red and green sequencesare forward and reverse shRNA sequences of β-catenin, and the bluesequence represents the loop sequence. Integration of the lentiviralconstruct was visualized by mCherry expression and clones were selectedbased on resistance to puromycin (FIG. 5B). Upon dox (dox) addition,both shRNAs efficiently downregulated β-catenin expression (FIG. 5C).The inventors used these cell lines to examine the stage-specific rolesof β-catenin during directed differentiation induced by Activin A andBMP4. Canonical Wnt signaling is essential for cardiac induction, sinceβ-catenin knockdown upon Activin A addition did not generatecTNT-expressing cells in the 19-9-11 ishcat-1 line (FIG. 5D), while theiscramble line showed no response to dox addition (FIG. 4F).Importantly, knockdown of β-catenin expression later in differentiationenhanced cardiogenesis (FIG. 5D). Similar results were observed for19-9-11 ishcat-2 line (FIG. 4G).

Example 5 Highly Efficient Generation of Human Cardiomyocytes Solely byModulating Gsk3 and Wnt Signaling

Our results and prior studies (Ren et al., J. Mol. Cell. Cardiol.51:280-287 (2011); Tran et al., Stem Cells 27:1869-1878 (2009); Wang etal., A.C.S. Chem. Biol. 6, 192-197 (2011); Willems et al., Circ Res.109:360-364 (2011)) indicate that induction of canonical Wnt signalingearly and suppression of canonical Wnt signaling at later stages ofdifferentiation enhance yield of cardiomyocytes. The inventors nextsought to determine whether modulating Gsk3 and canonical Wnt signalingalone was sufficient to induce cardiogenesis. Undifferentiated hPS cellswere cultured in RPMI/B27-insulin medium containing CH. CH was removedby medium exchange at day 1 and dox was added to the medium at varioustime points between day 0 and day 4. At day 5, the medium was replacedwith RPMI/B27-insulin (FIG. 6A) and cardiomyocyte differentiation wasassessed at day 15 by the percentage and yield of cTNT and/or MF20expressing cells. In the 19-9-11 ishcat-1 line, 12 μM CH produced themost cTNT+ cells at day 15 without additional dox-induced β-cateninknockdown (FIG. 7A). The inventors then investigated the effect of thetiming of dox treatment following addition of 12 μM CH. Addition of dox36 hours following initiation of differentiation generated 95% cTNT+cells with yields of approximately 15 cTNT+ cells per input iPSC (FIGS.6B and 7B). A high yield of cTNT+ cells was also obtained using 19-9-11ishcat-2 and three additional hPS cell lines (IMR90C4, 6-9-9, and H9)transduced with inducible β-catenin shRNA ishcat-1 (FIG. 7C).

The optimal conditions illustrated in FIG. 4B, 12 μM CH followed by doxtreatment at 36 hours, yielded relatively pure (>90%) cardiomyocytesthat contracted as coordinated sheets in multiple (>50) independentexperiments, demonstrating consistency and reproducibility. Thesecardiomyocytes were maintained as beating cells in culture for more thansix months. The cardiomyocytes exhibited normal cardiac sarcomereorganization, demonstrated by immunofluorescent staining of α-actinin,MLC2a, and cTnT (FIGS. 6C, 7D, 7E, 7F and 7G). Scanning electronmicroscopy also identified cells with myofibrillar bundles andtransverse Z-bands (FIGS. 4D and 7H), and cells enriched in mitochondria(FIG. 6D). Intercalated disks with desmosomes (FIG. 7I), typical ofcardiomyocytes, were also observed. To provide an initial assessment ofthe functional competence of cardiomyocytes generated by manipulation ofcanonical Wnt signaling in the absence of growth factors, the inventorsperformed sharp microelectrode electrophysiological recordings at 29days post-addition of CH. Representative recordings of action potentialsare shown for a ventricular-like cardiomyocytes (FIG. 6E).Cardiomyocytes also exhibited rate adaptation, as evidenced by decreasesin action potential duration in response to stimulation at increasingfrequencies (FIG. 6F). The observed decreases in duration werecomparable in magnitude to those previously observed for hESC- andiPSC-derived cardiomyocytes (He et al., Circ. Res. 93, 32-39 (2003);Zhang et al., Circ. Res. 104:e30-41 (2009)). These results suggest thatthe ion channels and regulatory proteins involved in action potentialgeneration and regulation are normally expressed in cardiomyocytesgenerated by Wnt pathway manipulation. Together, these results indicatethat functional cardiomyocytes can be successfully generated from hPScells solely by manipulating canonical Wnt signaling in the absence ofexogenous growth factors.

Example 6 Molecular Characterization of Cardiomyocytes Generated byModulating Gsk3 and Wnt Signaling

To better understand the heterogeneity and maturity of cardiomyocytesgenerated via Wnt pathway manipulation, the inventors examined theexpression of genes involved in cardiomyocyte development, myofilamentprotein expression, and the proliferative capacity of CMs during aperiod of 60 days post-induction of differentiation. Molecular analysisrevealed dynamic changes in gene expression with the induction of theprimitive-streak-like genes T (Asashima et al., FASEB J. 23:114-122(2009)) and MIXL1 (Davis et al., Blood 111:1876-1884 (2008)) shortlyafter CH addition, and downregulation of pluripotency markers OCT4 andNANOG within 4 days (FIG. 8A). Expression of the cardiac transcriptionfactor NKX2.5 (Lints et al., Development 119: 969 (1993)) began at day 3and persisted throughout the 60 day experiment. ISL1, a gene that marksprogenitors of the secondary heart field in the early embryo (Bu et al.,Nature 460:113-117 (2009)), was also detected at day 3, but ceased byday 30. TBX5 (Bruneau et al., Dev. Biol. 211:100-108 (1999)), GATA4 (Kuoet al., Gene Dev. 11:1048-1060 (1997a); Kuo et al., Circulation96:1686-1686 (1997b)), and MEF2C (Edmondson et al., Development120:1251-1263 (1994)) are important regulators of cardiomyocytedevelopment and their expression has been used to directly convertfibroblasts into cardiomyocytes (Ieda et al., Cell 142:375-386 (2010)).These three genes were expressed at different time points followingβ-catenin knockdown (at 36 hr) and expression of these genes persistedfor the full 60 days of the experiment (FIG. 8A).

To quantitatively monitor the differential expression of myofilamentproteins involved in cardiomyocyte specification, the inventors profiledMLC2a and MLC2v expression 20, 40, and 60 days following induction ofdifferentiation. At day 20, very few cTnT+ cells contained detectablelevels of MLC2v, a marker of mature ventricular cardiomyocytes (Francoet al., Anat. Rec. 254:135-146 (1999); Kubalak et al., J. Biol. Chem.269:16961-16970 (1994); Segev et al., Dev. Growth Differ. 47:295-306(2005)), while virtually all cTnT+ cells contained MLC2a, which isexpressed in mature atrial and immature ventricular cardiomyocytes(Kubalak et al., J. Biol. Chem. 269:16961-16970 (1994)) (FIG. 8B). Byday 60, greater than 50% of the cTnT+ cells expressed MLC2v while thepercentage of cTnT+ cells expressing MLC2a decreased to less than 80%,suggesting maturation of a population of ventricular cardiomyocytes.Smooth muscle actin (SMA) is expressed in the earliest embryoniccardiomyocytes but not in more mature cardiomyocytes (Matsui et al.,Dev. Dynam. 233:1419-1429 (2005); Nakajima et al., Develop. Biol.245:291-303 (2002); Ruzicka and Schwartz, J. Cell Biol. 107:2575-2586(1988); Sugi and Lough, Dev. Dyn. 193:116-124 (1992)). At day 20,greater than 80% of cTnT+ cells also expressed SMA, but by day 60 lessthan 15% of the CMs were SMA+ (FIG. 8C). Another hallmark of CMmaturation is the loss of proliferative capacity. To assess cellproliferation, the inventors quantified Ki67 staining and BrdUincorporation in MF20+ cells generated by the protocol illustrated inFIG. 6A. The percentage of proliferating cardiomyocytes decreased fromabout 25% at day 20 to less than 10% at day 60 (FIG. 8D). Takentogether, these results indicate cardiomyocytes generated by WNT pathwaymodulation transition from an early phenotype to a more mature stateduring culture.

Example 7 Induction of TGFβ Superfamily Signaling by Gsk3 Inhibitors

To determine whether canonical Wnt signaling requires TGFβ superfamilysignaling to induce cardiogenesis, the inventors quantified cTnT+ cellgeneration in 19-9-11 ishcat-2 cells when Activin A and BMP4 signalingantagonists were presented during the first 24 hr of cardiomyocyteinduction with 12 μM CH. All samples were treated with dox at 48 hours.SB431542 (SB), an inhibitor of the Activin A receptor-like kinase ALK5(Inman et al., Mol. Pharmacol. 62:65-74 (2002)), completely blockedcardiomyocyte specification at concentrations greater than 2 μM (FIG.9A). Addition of DMH1, which inhibits the BMP ALK2 receptor (Hao et al.,A.C.S. Chem. Biol. 5:245-253 (2010)), also decreased the percentage ofcTnT+ cells in a concentration-dependent manner (FIG. 9A). To furtherinvestigate the role of TGFβ superfamily signaling in Wntpathway-mediated cardiogenesis the inventors assessed Smad1/5 and Smad2phosphorylation, downstream of BMP4 and Activin A signalingrespectively. As expected, substantial Smad1/5 and Smad2 phosphorylationwas detected in cells that had been treated with Activin A and BMP4(FIGS. 9B and 10A). CH treatment resulted in Smad1/5 and Smad2activation at levels comparable to those induced by Activin A and BMP4.Smad1/5 phosphorylation was strongly attenuated by DMH1 while SBdiminished Smad2 phosphorylation. Interestingly, endogenous BMP2/4 wasdetected in undifferentiated hPS cells and cells following CH treatment(FIGS. 9B and 10A). Gene expression analysis revealed that BMP2 and BMP4were gradually upregulated upon CH treatment and persisted throughoutthe differentiation process while a transient upregulation upon CHtreatment was observed for NODAL expression (FIG. 9C). These resultsindicate that Activin/Nodal and BMP signaling are necessary forcardiogenesis induced via Wnt pathway regulators and suggests that thissignaling may result from endogenous Nodal and BMPs produced duringdifferentiation.

Example 8 Differentiation of hPS Cells to Cardiomyocytes in FullyDefined Conditions via Small Molecule Modulation of Gsk3 and WntSignaling

While shRNA inhibition of β-catenin provides specific and faciletemporal regulation of canonical Wnt signaling, this method requiresgenetic modification of the hPS cell line which limits its potentialclinical utility. The inventors next used the mechanistic insight thesemodified lines provided regarding the sufficiency of canonical Wntsignaling in cardiomyogenesis to develop a completely defined, growthfactor free method of efficiently generating cardiomyocytes fromunmodified hPS cell lines using small molecules. First, 19-9-11 iPSCswere maintained in mTeSR1 on Matrigel for five days, and then the mediumwas switched to RPMI/B27-insulin containing 12 μM CH. IWP4 and IWP2,which prevent palmitylation of Wnt proteins by porcupine therebyblocking Wnt protein secretion and activity (Chen et al., Nat. Chem.Biol. 5:100-107 (2009)), were used to inhibit Wnt signaling. Addition of5 μM IWP4 at day 3 resulted in optimal generation of cardiomyocytes(FIGS. 11A and 10B). Similar to results obtained with the 19-9-11ishcat-1 cell line, CH treatment of 19-9-11 cells alone only generated16% cTNT+ or MF20+ cells after 15 days, while adding 5 μM IWP4 or IWP2at day 3 increased this to 87% cTNT+ or MF20+ cells. Similar resultswere obtained when CH was replaced with other GSK3 inhibitors, includingCHIR98014, and BIO-acetoxime (FIG. 10C). In order to achieve fullydefined culture conditions, Matrigel was replaced with a defined peptideacrylate surface (Synthemax) during both hPS cell expansion anddifferentiation. 19-9-11 and IMR90C4 iPSCs plated on Synthemax platesand treated with CH and IWP4 also generated approximately 85% cTNT+ orMF20+ cells, comparable to the efficiency of differentiation observedafter CH treatment followed by expression of β-catenin shRNA (FIG. 11B).Thus, optimization of canonical WNT signaling via stage-specificaddition of small molecule agonists and antagonists produces high yieldsof functional cardiomyocytes in a completely defined, growth factor freeculture system.

The invention has been described in connection with what are presentlyconsidered to be the most practical and preferred embodiments. However,the present invention has been presented by way of illustration and isnot intended to be limited to the disclosed embodiments. Accordingly,those skilled in the art will realize that the invention is intended toencompass all modifications and alternative arrangements within thespirit and scope of the invention as set forth in the appended claims.

The invention claimed is:
 1. A method for generating a population ofhuman cardiomyocyte progenitors from human pluripotent stem cells,comprising: (i) activating Wnt/β-catenin signaling in human pluripotentstem cells by culturing the human pluripotent stem cells in the presenceof a Gsk3 inhibitor to obtain a first cell population; (ii) culturingthe first cell population for a period following the end of theactivating step; and (iii) inhibiting Wnt/β-catenin signaling in thefirst cell population while culturing the first cell population afterstep (ii) to obtain a second cell population comprising humancardiomyocyte progenitors, wherein each culturing step free of exogenousgrowth factors.
 2. The method of claim 1, wherein inhibiting Gsk3comprises contacting the pluripotent stem cells with a small moleculethat inhibits Gsk3, wherein the small molecule is selected from thegroup consisting of CHIR 99021, CHIR 98014, BIO-acetoxime, BIO, LiCl, SB216763, SB 415286, AR A014418, 1-Azakenpaullone, andBis-7-indolylmaleimide.
 3. The method of claim 2, wherein the smallmolecule is selected from the group consisting of CHIR 99021, CHIR98014, and BIO-acetoxime.
 4. The method of claim 1, wherein inhibitingGsk3 comprises RNA interference knockdown of Gsk3.
 5. The method ofclaim 1, wherein inhibiting Gsk3 comprises overexpression of a dominantnegative form of Gsk3.
 6. The method of claim 1, wherein activating theWnt/β-catenin pathway signaling comprises overexpressing β-catenin inthe cultured pluripotent stem cells.
 7. The method of claim 1, whereininhibiting the Wnt/β-catenin signaling in the first cell populationcomprises contacting the first cell population with a small moleculethat inhibits Wnt/β-catenin signaling.
 8. The method of claim 7, whereinthe small molecule stabilizes axin and stimulates β-catenin degradation.9. The method of claim 8, wherein the small molecule that stimulatesβ-catenin degradation and stabilizes axin is XAV939.
 10. The method ofclaim 7, wherein the small molecule prevents palmitylation of Wntproteins by porcupine.
 11. The method of claim 10, wherein the smallmolecule that prevents palmitylation of Wnt proteins by porcupine isIWP2, IWP4, or a combination thereof.
 12. The method of claim 1,wherein, in step (iii), inhibiting the Wnt/β-catenin signaling comprisescontacting the first cell population with at least one antibody thatblocks activation of a Wnt ligand receptor.
 13. The method of claim 12,wherein the at least one antibody binds to one or more Wnt ligand familymembers.
 14. The method of claim 1, wherein, in step (iii), inhibitingthe Wnt/β-catenin signaling comprises reducing β-catenin expression inthe first cell population.
 15. The method of claim 14, wherein reducingβ-catenin expression comprises expressing shRNA for β-catenin in thefirst cell population.
 16. The method of claim 1, further comprisingculturing the second cell population after ending the inhibition ofWnt/β-catenin signaling initiated during step (iii) to obtain a cellpopulation comprising cardiomyocytes.
 17. The method of claim 16,wherein the culturing in steps (i)-(iii) comprises culturing in a mediumcomprising RPMI base medium, wherein at least 70% of the cells in thecell population comprising cardiomyocytes are cardiac troponinT(cTnT)-positive, and wherein no cell separation or selection step wasused to obtain the cell population comprising cardiomyocytes.
 18. Amethod for culturing human pluripotent stem cells to obtain a populationof human cardiomyocytes, the method comprising the steps of:sequentially inhibiting Gsk3 in the human pluripotent cells and theninhibiting Wnt signaling in the Gsk-inhibited cells; and culturing thesequentially inhibited cells in a culture medium to form adifferentiated cell population comprising cardiomyocytes, wherein theculturing is free of exogenous growth factors.